1. Field of the Invention
This invention relates to an improved low cost open loop current stabilization circuit for starting and operating ultra violet (UV) lamps, and more particularly for starting and operating low pressure germicidal UV lamps used in water sterilization apparatus.
2. Description of the Prior Art
The performance of water sterilization apparatus utilizing low pressure germicidal UV lamps is directly related to absolute current flow in the UV lamp.
The ballast circuits used in UV water sterilizer applications today are generally modified versions of ballasts originally developed for fluorescent lighting applications. These ballasts generally have no current control function, and this results in the current flow in the low pressure mercury UV lamp varying with changes in the input supply voltage. In the case of simple magnetic ballasts, current flow in the low pressure mercury UV lamp will also vary with frequency of the input AC power.
The reliability of many of the ballasts presently used in UV water sterilization apparatus, especially when operating from 240 volt AC input power and utilizing short arc length lamps, has not been satisfactory.
Various types of ballast circuits utilizing the frequency modulation current control technique are well known in the art.
U.S. Pat. No. 2,928,994 by Widakowitch shows a saturable core variable frequency DC powered inverter whose frequency varies as a function of the DC supply voltage to maintain the current flowing in the lamp relatively constant.
U.S. Pat. No. 4,060,751 by Anderson and U.S. Pat. No. 4,060,752 by Walker both describe inverter circuits in which the switching elements are commutated when the instantaneous current exceeds a predetermined level. The lamp is coupled to the inverter output with a series inductor capacitor network with the lamp connected across the capacitor element. Before ignition the lamp appears as a high impedance shunted across the capacitor. On power up the inverter output sees only the series inductor capacitor network and is forced to operate at the resonant frequency of the series inductor capacitor combination. As the resonance voltage builds, the point at which lamp ignition is initiated is reached and the negative impedance of the ignited lamp effectively shunts the series capacitor. After the lamp is ignited, the inverter switches are commutated when the instantaneous lamp current exceeds the predetermined current threshold. As the rate of current rise increases, the predetermined current reference point will be reached more quickly increasing the frequency of oscillation of the inverter and reducing the current flow through the series combination of the ballast inductor and UV lamp. As the rate of current rise decreases the amount of time to reach the predetermined current switching threshold is extended, lowering the frequency of oscillation and allowing more current to flow through the series combination of the ballast inductor and UV lamp.
U.S. Pat. No. 4,498,031 by Stupp, U.S. Pat. No. 4,585,974 by Stupp, U.S. Pate. No. 4,698,554 by Stupp and U.S. Pat. No. 4,700,113 by Stupp disclose a ballast circuit consisting of driven push-pull inverter circuit driven by a voltage controlled oscillator whose output is divided by two with flip-flop circuit to insure symmetrical drive to the inverter switches. In all cases a closed loop current sensing circuit controls the frequency of the voltage controlled oscillator. The use of smaller filter capacitors to filter the pulsating DC output of the AC to DC rectifier to improve the circuit power factor is claimed and implication of improved lamp current crest factor though not specifically mentioned is obvious from the operational description. The use of analog, digital and hybrid analog digital implementations for the oscillators is discussed.
U.S. Pat. No. 4,717,863 by Zieler describes a ballast circuit with a driven push-pull inverter and non resonant output network. The output frequency of the inverter is at the lower limit of operation on start up to provide maximum preheat current to the lamp filaments.
Control of the lamp current after ignition using closed loop circuits based on lamp current or measuring lamp output with a photo-detector circuit are described. Additionally, an interface to allow the desired output level to be programmed by an external computer input is disclosed.
U.S. Pat. No. 4,727,470 by Nilssen describes a ballast circuit utilizing a self oscillating half-bridge inverter topology using a saturable core transformer in the feed back Idop. The lamp is connected to the inverter output across the capacitor of series inductor capacitor network. On start up the lamp appears as a high impedance parallel with the capacitor element of the series inductor capacitor output network and the inverter oscillates at the resonant frequency of this network. As the resonance voltage builds, the lamp is ignited and the capacitor is effectively shunted out of the series inductor capacitor network by the negative impedance of the lamp. The operating frequency of the inverter when the lamp is ignited is determined by the saturation flux density of the saturable core transformer feedback element. A technique for modifying the saturation point of the saturable core feedback element in both closed loop and open loop current control topologies is described. Improved powerfactor and lamp current crest factor are both claimed.